Bipolar transistors are electronic devices with two p-n junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, two p-n junctions, i.e., the emitter-base and collector-base junctions, are separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar transistor action”.
If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because the mobility of minority carriers, i.e., electrons, in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher frequency operation and higher speed performances can be obtained with npn transistors. Therefore, npn transistors comprise the majority of bipolar transistors used to build integrated circuits.
As the vertical dimensions of bipolar transistors are scaled more and more, serious device operational limitations have been encountered. One actively studied approach to overcome these limitations is to build transistors with emitter materials whose band gap is larger than the band gap of the material used in the base. Such structures are referred to in the art as ‘heterojunction’ transistors.
Heterostructures comprising heterojunctions can be used for both majority carrier and minority carrier devices. Among minority carrier devices, heterojunction bipolar transistors (HBTs) in which the emitter is formed of Si and the base of a silicon germanium (SiGe) alloy have recently been developed. The SiGe alloy is narrower in band gap than silicon.
The advanced SiGe bipolar and complementary metal oxide semiconductor (BiCMOS) technology uses a SiGe base in the HBT. In the higher-frequency (such as multi-GHz) regime, conventional compound semiconductors such as, for example, GaAs and InP, currently dominate the market for high-speed wired and wireless communication devices. SiGe BiCMOS promises not only a comparable performance to GaAs in devices such as power amplifiers, but also a substantial cost reduction due to integration of HBTs with standard CMOS, yielding the so-called “system on a chip”.
As silicon germanium (SiGe) heterojunction bipolar transistor (HBT) performance moves up over 200 GHz, it has become apparent that the avalanche degradation mechanism becomes the dominant reliability concern for SiGe HBT circuit applications. This is due to the fact that the high frequency performance of the bipolar transistor is achieved by vertical scaling of the device, which decreases the vertical depth of the junctions and increases the electrical field within the device. This high electrical field at the collector-base junction during operation generates high energetic carriers that can damage the insulating interfaces around the device's emitter and shallow trench isolation (STI) interfaces. Avalanche carrier related damages will decrease (or degrade) the device current gain in both forward and reverse active mode.
The avalanche degradation mechanism was recently discovered and it imposes a very big constraint for high frequency and high power performance of SiGe HBTs. See, for example, G. Zhang, et al., “A New Mixed-Mode Base Current Degradation Mechanism in Bipolar Transistors”, IEEE BCTM 1.4, 2002 and Z. Yang, et al., “Avalanche Current Induced Hot Carrier Degradation in 200 GHz SiGe Heterojunction Bipolar Transistors”, Proc. International Reliability Physics Symposium, pp. 339–343, 2003.
A sample avalanche degradation of a typical SiGe HBT is shown in FIG. 1. Specifically, FIG. 1 shows the current before the stress (T0) and after 3000 seconds (T1) avalanche stress for an IBM 200 GHz SiGe HBT with an emitter size of 0.8×0.8 μm2 stressed at VCB=3.0 V and IE=5.12 mA. Operation in the avalanche regime has become more and more important for SiGe HBTs in high frequency applications; See, for example, H. Li, et al., “Design of W-Band VCOs with High Output Power for Potential Application in 77 GHz Automotive Radar Systems”, IEEE GaAs Digest, pp. 263–266 (2003). VCB denotes the collector base voltage and IE denotes the emitter current.
Any methods to recover the avalanche degradation will greatly benefit the SiGe HBT circuit's performance and application range. However, there has not been any recovery method reported in the prior art to date because this degradation mechanism has only been fully investigated in the last year or so.
In view of the above, there is a need for providing a method to recover the avalanche degradation mentioned above in order to fabricate bipolar transistors, particularly SiGe HBTs, that can operate at the high frequencies currently required for the present generation of bipolar transistors.